This invention was made under a contract with the Department of the Army.
This invention relates to an improved log-periodic monopole antenna. The term "monopole" as used herein refers to an antenna having driven or radiating elements that are approximately one-quarter wavelength long at the operating or radiating frequencies and which are electrically isolated from an electrically conductive ground plane to produce a radiation pattern approximating that of dipoles.
The need for a very broad band vertically polarized HF (2-30 MHz) VHF (30-300 MHz) monopole antenna which can be mounted over a ground plane has long been recognized. Such antennas are useful for communications, direction finding, surveillance, and electronic countermeasures. While such performance can be obtained from the conventional log-periodic dipole antenna, the height of the latter at the low frequency end is impractical i.e., 250 feet at 2 MHz. In addition, the antenna must be elevated substantially above the ground to avoid periodic gain drop-out in which the radiation pattern breaks up at spaced frequencies across the band. The reduction in antenna height afforded by the monopole version is approximately 1/2 that of the dipole version as is well known in the art and is described in an article entitled, "Log-Periodic Monopole Array" by Berry et al, 1961 IRE International Convention Record, Part 1, Mar. 20-23, 1961, pages 76-85.
In order that any log-periodic antenna, including the monopole version, may produce backfire radiation patterns, the phase of an applied electromagnetic signal must be shifted 180.degree. between adjacent antenna elements. This has been accomplished in the past with a log-periodic monopole antenna by interspersing parasitic elements between the active or driven elements. Such an antenna is described in U.S. Pat. No. 3,286,268. While this design incorporates the necessary phase reversal between adjacent drive elements, the antenna has the disadvantage that the relative positions of the parasitic elements with respect to the active elements are very critical to the performance of the antenna. This not only places somewhat narrow limits on mechanical design tolerances but also requires that all radiating elements be rigidly mounted to maintain their relative positions during operation in order to insure acceptable performance characteristics. Furthermore, experience with this antenna indicates the need for improvement of repeatability and predictability of performance.
Another related prior art structure is described in a report entitled, "Bent Log-Periodic Zigzag Antenna" by J. G. Greiser, Supplement to Interim Engineering Report No. 4 for Broadband Wide Aperture Radio Location Antenna System, University of Illinois, May 31, 1962. In this structure, every other element functions as a radiator while intermediate elements are bent orthogonally to the radiating element and are used as phase-adjusting stubs. These stubs, being very close to the ground plane, behave as transmission lines, do not radiate, and provide the necessary phase reversal between adjacent elements. A difficulty with this antenna is its size in the horizontal plane. For example, while operating at a frequency of 2.0 MHz, the longest phasing stub is in the neighborhood of 12 feet. Other disadvantages are (i) the critical spacing between the antenna and ground plane required to maintain a good input impedance match, and (ii) the non-uniformity of tapers of radiating elements and phasing stubs resulting in increased difficulty in the design and fabrication of the antenna.
This invention is directed to a log-periodic monopole antenna which overcomes these disadvantages.